Consuls were the chief civil and military magistrates, elected through the assemblies by popular vote. Two annually elected consuls convened the senate and the curiate and centuriate assemblies.
Initially the office was only open to patricians until the Lex Licinia opened it to Plebeian candidates in 367 BC. According to the Lex Villia annalis passed in 180 BC which established minimum age requirements for all magistrate positions within the Cursus Honorum, Consuls had to be 42 years of age. Under normal circumstances, a Roman could only serve in such a capacity only once every ten years.
At the end of their annual term of service, Consuls would take the title Proconsul and generally serve as provincial governors. In the case of the death of a serving Consul, a Suffect Consul would be elected as a replacement for the remainder of his term. They were entitled to 12 Lictors as a symbol of their authority (or imperium).
In the imperial period, the defining terms of the consulship was far less strict. Length of service could be as short as only a few months, and any number of appointees could hold the post (a maximum of 25 men held the position under Commodus in 190 AD).
This allowed for a larger number of potential proconsular provincial appointments and military commands throughout the empire in addition to limiting the authority of any singular imperial rivals. Despite the reduction of their authority in comparison to the being the chief authorities in the Republican period, the consulship was still a vital magistracy in the empire.